Needles for application or insertion into an artery, vein or other blood vessel or cavity are utilized for the withdrawal of blood from a blood vessel, or body cavity, and/or the delivery of blood or other fluid to such blood vessel or body cavity. One common usage of such needles is in hemo-dialysis in which blood is removed from a patient, purified and returned to the patent. In dialysis and other medical procedures relating to the injection and/or withdrawal of fluids in the body, it is desirable and generally important to optimize the rate at which the blood or other liquid flows through the needle. The major limitation in the rate of blood flow is the size of the cross-sectional area of the needle, since the tubing used to carry the blood to the dialysis equipment is of significantly greater diameter than the needle. Generally, the larger the cross-sectional area of a needle is throughout its length, the greater the flow rate of the liquid therethrough. In fact, the rate of flow of a liquid through a needle is proportional to the radius (r) of the needle raised to the fourth power and inversely proportional to the length of the needle, according to the Hagen-Poiseuille's equation, set forth EQU Q=(.pi.r.sup.4 .DELTA.P)/(8 .mu.L)
Q=flow PA1 r=radius of conduit PA1 .DELTA.P=pressure gradient PA1 L=length of conduit PA1 .mu.=constant
The pressure gradient (.DELTA.P) that can safely be applied to the blood is limited by the capacity of the blood cells to withstand the pressure without hemolyzing (rupturing). High negative pressure results in damage to the cell membrane and hemolysis. Therefore, the only means to optimize flow through the needle is to modify the radius and the length of the needle. The length is difficult to modify because of certain inherent factors such as the distance of the target blood vessel to the surface of the skin and the best or preferred angle for inserting the needle into the blood vessel. Therefore, the best and most common parameter of the above equation which is modified to increase the fluid flow through a needle is the radius.
In the case of kidney dialysis, the criticality of the rate of blood flow for dialysis patients can be better understood when it is considered that each adult kidney patient requiring dialysis must receive dialysis treatment three times a week for his or her life, and each such treatment requires typically approximately four hours to complete. Thus, ignoring other factors, according to Poiseuille's equation, the doubling of the internal diameter of a needle throughout its length would result in a sixteen-fold increase in the rate of blood flow, which, in turn, would result in a sixteen-fold time reduction for each dialysis treatment. Accordingly, a reduction in treatment time, in turn, would reduce the cost of dialysis treatment substantially. Even a 10 percent increase in needle diameter results in approximately a 46 percent (1.1.sup.4) increase in flow rate. Prior art dialysis needles have an internal diameter in the range of approximately 1.6 to 2.2 mm (generally, 16 gauge needles are used), which is generally larger than those used for hypodermic injections. Such needles are also generally manufactured with ultrathin wall thicknesses of 0.05 to 0.1 mm to obtain the smallest possible outer diameter with the greatest possible inner diameter.
Of course, there are inherent problems involved with using needles having particularly large cross-sectional diameters (e.g. large gauge needles). For example, large gauge needles tend to be more painful to a patient than smaller needles. Also, the larger puncture wound caused by a large gauge needle requires greater healing time. Further, larger gauge needles have a greater risk of infection as a result of the aforementioned larger hole and longer healing time. There is also a psychological factor involved, in that persons who are afraid of needles tend to have a greater fear of larger gauge needles.
Further, in this connection, it is also known that skin and blood vessels are flexible and elastic and can stretch to some degree when stretched at a reasonably slow rate. Needles used for dialysis, except for the beveled point used to cut a hole in the skin and blood vessel, are substantially of a uniform diameter throughout their length. Thus, prior art needles do not effectively rely on the ability of the skin and blood vessel to stretch to permit the insertion of a needle of relatively large diameter into a relatively small hole, but instead, cut a relatively large hole.
In addition, prior art needles, particularly those intended to remain in place for extended periods of time, have certain inherent problems with respect to their ability to remain fixed in place when inserted in the intended blood vessel or body cavity. In particular, such needles must be held in place by adhesive or other securing means; otherwise, the needle is easily displaced because the smooth sides thereof do not create enough resistance to be held in place.
The present invention overcomes the drawbacks of the prior art needles, providing a needle having a large internal diameter capable of being inserted into a blood vessel without puncturing a large hole in the skin and blood vessel or body cavity, which needle is securely disposed in a predetermined area.